http://www.naturalhealth.net.nz/ebooksnl/bl/titles/secretlangoflife/plantshavesenses.htm
Plants are bursting with movement. They are rich in sensation,
and respond to the stimulation of the Surrounding world every
moment of their active lives. They can send messages to one another
about overcrowding or a threatened attack by a new pest. Within
each plant there is ceaseless activity as purposive as that in
an animal. Many of them share hormones that are remarkably similar
to our own. Their senses are sophisticated: some can detect the
lightest touch (better than the sensitivity of the human fingertips),
and they all have a sense of vision.
Trees manage to grow in well-spaced patterns, as a walk through
woodland will confirm. They employ mechanisms designed to prevent
overcrowding, which would lead to competition for food, light,
and water. Not only can plants communicate an attack by pests
to other plants in the neighbourhood, but they can react to disease
by chemical responses which parallel some of those seen in animals.
Plants have great regenerative powers, and the way they heal themselves
shows immense coordination of cellular growth. A tree from which
a branch has been cut covers the site with wound tissue and makes
good the damage. If you do not cut down a branch, then the tree
may well do that for itself.
Trees have the ability to configure their outline during their
lifetime. For example, they can shed their branches to maintain
their equilibrium. An even more remarkable ability is reported
by Bill Vinten in Suffolk, who reports that a tree which was partly
dislodged by a gale has altered its branches to regain its balance.
He observed that the tree had been left leaning down wind after
the storm. Over the following years, no branches were lost from
the tree, but those that remained have grown round to restore
the tree's centre of gravity. We have no knowledge of how a tree
does this, and the maintenance of the outline of a tree is clearly
a result of its sensory awareness and is worthy of further study.
Plants have much in common with animals. The essential difference
is that a green plant can capture sunlight and use its energy
to power its life processes. The light from the sun is used by
the plant cells to do something science cannot imitate: they take
molecules of carbon dioxide and water, and fit them together to
make carbohydrates. As long as plants are in the light, they will
keep doing this. The simplest carbohydrate molecules are sugars,
but; among the more complicated carbohydrates is cellulose. Cellulose
does not dissolve in water, as sugars do, so it has to be laid
down where it cannot harm living cells. As a rule, the cellulose
is deposited around the outside of plant cells. This means that
each plant cell is surrounded by a cellulose wall, and is trapped
inside this insoluble box. Whereas typical animal cells can stretch
and change shape, can expand and contract and can easily divide
into two, mature plant cells are surrounded by a stiff capsule
of cellulose and are far less mobile.
The growing tip of a plant is made of thin-walled cells which
can still divide. The direction of growth is normally towards
light (plants can, to that extent,'see') and upwards, away from
the ground (they can sense gravity). As the cells mature they
lay down their cell walls. Cellulose is the typical deposit in
plant cell walls, though some plants produce other substances.
Lignin, for instance, is widely found in woody tissues. Structures
made of silica glass are found in some cell walls, which is why
a blade of grass can cut the skin like a saw. It is the wall of
a plant cell which prevents such plants from moving around. They
are rooted to the spot, taking in water that evaporates from the
leaves. This continuous current of water passing up along the
stem carries nutriment through the body of the plant. The movement
of water is called transpiration, and it is vital for a plant's
growth.
The evaporation of water from a plant is regulated by pores (stomata)
which can open and close as the plant dictates. They also control
the passage of air and carbon dioxide through the network of cells
within the leaf. Each stoma is controlled by two guard cells which
can open and close the aperture between them. The chemistry is
complex, but when the guard cells enlarge, so that the pore opens,
potassium chloride migrates into the cells and starch breaks down.
When the processes reverse, and potassium chloride is released
from the cell as starch reforms, the guard cells lose volume and
the pore closes. The guard cells do not simply control the pores,
but sense what is going on around the plant. First, they react
to the intensity and the quality of light. Second, the stomata
can detect the chemical nature of the atmosphere, responding to
levels of carbon dioxide and other gases. Third is their ability
to respond to physical stimuli that affect the leaf, like vibrations
and movement caused by wind, and their fourth sense is of substances
produced by organisms on the leaf surface. As Terry Mansfield
of the University of Lancaster has pointed out, these correspond
to four of the classic senses: sight, smell, touch and taste.
From pages: 188 - 189
Although green plants have no nervous system, they can transmit
messages through the length of the plant body. It has long been
thought that the stomata open and close solely in response to
what they sense, but we now know that they can also be controlled
from as far away as the tips of the roots. During periods of drought,
the water flow up a plant diminishes and the stomata close down
to minimise water loss. It has long been believed that it was
the closure of these pores in the leaf which reduced the water
flow. In the 1980s it was discovered that the stomata start to
close down the moment the roots detect dry soil, and long before
there is any change in the water reaching the leaves. The plants
are anticipating a threat before it arises. The mechanism seems
to be some form of chemical signal which the plants can send to
the leaves, and which leads to the closure of the stomata before
the plant experiences water loss.
One of the simplest methods of demonstrating this effect is the
split-root experiment. A plant is induced to share its root system
between two pots, which can be independently supplied with water.
If the soil in one of the pots dries out, the stomata over the
whole plant tend to close. This occurs even if there is a plentiful
supply of water to the second pot. The crucial role of the roots
can be demonstrated by watering the dry soil, for the stomata
immediately open. Confirmation is obtained by cutting off the
roots to the dry pot. As soon as the roots are detached, the signalling
system is severed and the stomata open. The control of the stomata
by the roots may be by means of abscisic acid, a hormone which
can cause stomata to close if present in very small amounts. One
part of abscisic acid in a billion parts of water is enough to
make them shut. Analysis of roots from split-root plant experiments
substantiates the possibility, for there is always much more abscisic
acid in the dry roots than in the moist ones.
Plants are subject to a great range of stimuli, and have fine
tuned senses to optimise their behaviour. Inside a plant there
is ceaseless activity. Microscopic particles within each cell
are moving around to catch the light to the best advantage, and
cytoplasm inside the cells is streaming from one place to another.
Water is drawn up through the stem, and elaborated foodstuffs
pass down like a nourishing blood supply. A plant in woodland
may seem static to the casual observer, but inside it is a hive
of activity.
From pages: 190 - 192
[The Venus Fly Trap] seems to be the closest to an animal, for
it has jaws which snap shut in one-third of a second, and teeth
which hold the prey. . . Each trap is composed of a leaf divided
into two oval portions, which open like a butterfly's wings. On
the glandular inner surface of each lobe are three fine spiny
hairs, and around the far edge are interlocking teeth like those
of a man-trap. A slight movement against one of the hairs triggers
the trap into action: the two halves spring towards each other
so fast that they prey cannot escape. . . Was the effect purely
mechanical, or might there be a nervous response within the plant?
Sir John Burdon-Sanderson (1828 - 1905) hitched the leaves up
to the apparatus that he was using to study how nerves send signals
to muscles, and soon found an electrical signal in 1873. The electrical
impulse was recorded immediately after the sensitive hairs were
touched, and before any movement was detected. Here was a series
of three events: stimulation of the hair, electrical impulse,
and closure of the trap. This is very similar to the way an animal
responds to a stimulus. . .
If the chemical change were produced by the stimulation of one
of the trigger hairs on a fly-trap, we would expect it to be detectable
as an electrical response. Just because we measure a peak of electrical
activity does not necessarily mean that there is a nervous reaction;
we could merely be measuring the results of a chemical change
inside the cells. This objection was raised, at the time by Julius
von Sachs, (1832 - 97) an eminent German botanist, who concluded
that there was no true nervous activity in the fly-trap. He had
two main reasons for drawing this conclusion. First, the speed
of the impulse was too slow. Impulses can travel along animal
nerves at thousands of centimetres per second, while the rate
in the Venus fly-trap, was only 20 cm (8 in) per second. And second,
there are no nerve cells in the fly-trap. Surely, von Sachs argued
if there were nerve impulses, they must have nerves along which
the impulses travel. This scepticism resulted in a lack of further
interest in the study of plant movement and the subject lay dormant
for a century. But in the 1960s, US scientists started to study
how impulses are transmitted by cells, and similarities between
plants and animals started to emerge.
Life on earth first developed in the salt water of the seas, and
one of the fundamental mechanisms of any living cell is a way
of controlling the level of salt - sodium chloride. Keeping the
sodium ions from salt water at bay is done by restricting their
movement in and out of a living cell. The cell is covered with
a thin skin, the cell membrane, which is a fatty layer insulating
the inside of the cell from the outside environment. Because of
the ions inside the cell there is a negative electrical charge
(about one-tenth of a volt). The central layer of fat in the cell
membrane acts as an electrical insulator. Ions can pass this barrier
only if tiny apertures open to allow them through, and these gates
are used by the cell to regulate the passage of ions through the
membrane.
From pages: 192 - 194
If an impulse passes along a nerve in an animal, an advancing
wave of gates opens to allow sodium ions into the nerve and potassium
ions out. Now that we can study plant cells, the same mechanism
has been detected in them. The most interesting discovery of all
was that plant cells, like animal nerves, can manifest a receptor
potential before the action potential itself. The action potential
is the electrical signal that induces a response in a living cell,
the receptor potential is the signal created before that - when
the stimulus itself is detected. The receptor potential results
from the hair-cells in the fly-trap being touched. This stimulus
is translated into an action potential only if it is strong enough.
So a small stimulus producing a modest receptor, potential, may
be insufficient to cause the action potential to be triggered;
in which case, the trap stays open. A strong: stimulus will create
a receptor potential sufficient to generate an action potential,
which closes the trap. But what happens with medium strength stimuli?
If a stimulus is below the threshold, the trap remains open; but
if several more small stimuli are received, the hair-cell will
still generate an action potential. It is as if the cell is remembering
the stimuli and adding them up, which suggests that there is a
kind of memory within the plant-cell. This means that a tiny fly
will not ordinarily be trapped, but a slowly moving insect, which
produces only the slightest touch, will be trapped if it causes
gentle movements of a hair cell. As a rule there needs to be a
second stimulation within half a minute for the two to be associated.
The sequence becomes complex:
1. Movement of a hair-cell, if slight, may be ignored; if sufficient,
it causes a receptor potential to fire.
2. If this is small, it is ignored; if it is sufficiently strong,
an action potential will fire and an electrical charge will pass
across the open trap.
3. If the action potential is insufficient, the trap remains open;
if it is strong enough, the trap will snap shut.
4. A later stimulation of a hair-cell may be too weak to generate
a response. If a further stimulation within half a minute is enough
to trigger a potential, the trap will close.
There are several stages of information processing in the Venus
fly-trap. The timing of the responses, together with their nature,
shows that they are based on mechanisms much like those in animals.
From page: 195 Were the plants not confined by their stiff cell
walls of cellulose, they might match animals for speed. The study
of electrical activity in plants shows that they can sense touch,
and respond to the stimulus, in a coordinated and appropriate
manner. Clearly, they process data. They do no more than they
need to, but what they do is well adapted to the demands of their
daily lives.
From pages: 197 - 200
Mechanical Movement Some of the movements in the plant
kingdom result from harnessing the mechanical properties of matter.
One of the earliest to appear in the literature of science was
the awn of a grain of oats, a flexible spine at the end of each
grain which twists and untwists according to the humidity of the
air. Robert Hooke (1635 - 1703), the natural philosopher who is
commemorated by Hooke's Law, used a wild oat awn to measure humidity
and drew the apparatus in his pioneering book Micrographia,
, first published in 1665. This marked the invention of the
hygrometer. Some seeds harness the movement induced by changes
in moisture to propel themselves into the ground. The storksbill
Erodium, has spiral awns which coil and uncoil with changes
in humidity. They propel the seeds across the surface of the earth
until they encounter a crevice, and then the twisting movement
helps to screw the seed deep into the earth where it can germinate
in seclusion.
The drying out of fern sporangi is used to propel spores away
from the plant like stones from a catapult. These sporangia are
fringed by a layer of peculiar thickened cells, the annulus. Most
textbooks explain its function incorrectly. They tell how the
drying of the sporangium causes the cells of the annulus to contract
suddenly, breaking open the capsule and scattering the spores
far and wide. That is not what happens. In reality, the drying
causes the capsule to break as the catapulting collar slowly bends
backwards. As water evaporates from the cells they contract even
further, until the whole structure is bent back upon itself. Suddenly,
the partial vacuum inside each cell of the annulus is more than
can be endured. The cell walls rupture, air rushes in, the shock-wave
causes all the other cells to rupture in sequence, and the annulus
springs back to its former position with a dramatic jerk. The
effect is to launch the spores at high speed, all in the same
direction, well away from the fern on which they formed. They
travel far enough to reach currents of free air, and this completes
their distribution. . .
Arceuthobium, has a more dramatic method. The fruits are
filled with seeds lying in a liquid. The pressure builds up as
the seeds mature, until the end of the fruit ruptures and the
seeds are squirted out at speed up to 100 km/h (about 60 mph).
They can easily reach the next tree, and this form of parasitism
has caused heavy losses to the timber trade.
Many flowers have a spring-loaded mechanism. The leguminous flowers
(such as peas, beans and alfalfa) hold their stamens between paired
petals which form a keel at the base of the flower. If small insects
alight, nothing happens. It takes a bee of the right size to bear
down on the keel. When that happens, the petals burst open and
the stamens shoot upwards like an uncoiled spring, dusting the
insect with pollen. . .
Flowers that remain unpollinated by insects (those grown in greenhouses,
for example) sometimes spring their anthers without any outside
interference. It is as though they have retained memory of what
they are meant to do. There are aspects of plant behaviour that
seem to suggest that a plant can store a memory of an earlier
experience. Plants have been shown to remember earlier traumas
- they can recall being wounded on one side, and compensate for
the damage by later growth. If you have dandelions in a mown lawn
you may observe that they flower almost prostrate on the ground,
as though they have learned that a raised profile will lead to
them being cut off in their prime. Equally interesting is the
fact that plants can distinguish between one stimulus and another.
If a sensitive plant is repeatedly stimulated by touch it will
eventually fail to respond. If another form of stimulus is applied
(an electrical stimulus, say) it will immediately respond to that.
Plants clearly have the means to tell different types of signal
apart. More research is needed into these phenomena, which suggest
the plants are more alive to their surroundings than is widely
believed.
From pages: 201 - 203
One plant, above all, is known as the sensitive plant. It closes
its leaves in a second, and has long been known for this remarkable
phenomenon . . . This is the Mimosa pudica, though there
are several other species which are just as touchy . . . a train
rattling along the track, can send sufficient stimulus to have
the Mimosa plants closing up like umbrellas.
There are two remaining problems with Mimosa pudica., One
of these is where its sensitivity resides. There are no sensory
hairs, and no specialised cells that seem to be specifically adapted
to detecting pressure or touch. The cells of the leaf seem much
the same as cells in any other plant. And, though there are several
other sensitive species of Mimosa, others show no such movement
at all, Mimosa dealbata looks very similar, for example,
and shows similar cells under the microscope, but lacks any sensitive
movement; The second major problem is the purpose that the collapse
of the leaves might serve The rock-rose moves its anthers to protect
its pollen, the fly-trap springs its leaves shut to capture prey.
Without these mechanisms, the plants would find it hard to survive
where they grow. This does not appear to be the case for Mimosa
pudica. Many theories have been put forward -protection from
the sun, avoiding being grazed by passing herbivores, and so forth
- but they would apply equally to any other species. If those
were the reasons, we would have leafy plants drooping and closing
wherever we looked. We don't, of course. These mysteries remain.
The earliest research into the mechanism behind the sensitive
plant showed that contact produced a collapse in the cells that
ordinarily hold the leaves erect. As water passed rapidly out
of these supporting cells, it was believed that they collapsed
mechanically inwards and allowed the leaves to droop. A few years
after the historical experiments by Burdon-Sanderson, who measured
electrical responses in the Venus fly-trap, a German physiologist
- Karl Kunkel of Heidelberg - described similar electrical impulses
in Mimosa pudica as the leaves moved, This failed to trigger
an upsurge in research, because it was concluded that the electrical
activity was the consequence of the collapsing cells, and not
the cause.
In recent years we have begun to find many other clues to the
nature of the movement, and some of them point towards a distinctly
animal-like series of mechanisms. There are long, thin cells inside
the tissues which conduct sap, and some people have likened them
to a kind of nerve fibre. Tannin was discovered within the cells,
and tannin is a concentrated source of potassium ions, which is
important for movement in animals. It has been discovered that,
when the supporting cells collapse, there is a sudden surge of
potassium through the cell membranes which causes them to lose
water and collapse. Tiny vacuoles, fluid-filled spaces within
each cell, have been found which have the ability to squirt water
out of a cell at speed. More surprisingly, there are known to
be fine fibrils inside the cells, which seem to be able to contract
like muscle cells in animals. Why the sensitive species of Mimosa
move so rapidly, while other very similar-looking species are
unmoved by the sense of touch, is still unknown. Meanwhile, it
is clear that these curious plants have mechanisms which are parallel
to those we see in animals. They have a remarkable sense of touch,
and respond much as animals would respond.
Refined senses
The sense of touch is highly influential in the life of many flowering
plants. It is at its most highly developed in climbing plants,
which have developed an extraordinary sense. Tendrils are highly
adapted organs used by many plants to support them as they grow.
Most tendrils are a few centimetres in length, but those of the
grape vine Vitis can measure 50 cm (20 in). Normally, the
tendrils move slowly round in an oval pathway as the plant grows
up, as though searching for a point of contact. On a speeded-up
time-lapse film the effect looks startlingly like a blind creature
feeling its way. There is a sense of orientation to these searching
movements. For example, the pea plant moves its tendrils in an
ellipse so that the long axis of the ellipse is always at right
angles to the sun. If an outstretched comes into contact with
a solid support, it will slowly grow to enclose it and thus to
support the plant. This response is called contract coiling.
From pages: 204 - 206
Pea plants are convenient for casual study, for their tendril
can store the memory of a stimulus. If a pea tendril is stroked
it will start to curl, though if the plant is chilled this responses
does not occur. The memory remains, however, and if the plant
is later allowed to warm up, the tendril then curls as if recalling
or having stored the earlier stimulus. The tip of a pea tendril
grows into a sharp little hook, helping to attach it to its support.
If you stroke an outstretched pea tendril you will see it start
to coil within a minute or so. Try it at night and nothing happens.
The tendrils need to be in the light before they will respond
to stroking. They can store the effect of the stimulus for more
than an hour and, if brought into the light 90 minutes after the
stimulus, they will start to coil as though they had just been
touched . . .
In plants like the Virginia creeper Parthenocissus . .
. [it is not clear, but it may be that Ford is referring to Parthenocissus
here] . . . if even a single touch-cell is stimulated, the effect
is transmitted to all other cells in the tendril, so coiling starts
simultaneously all along its length. These cells can clearly communicate
with their neighbours. The sense can be more highly developed
that the sense of touch in humans. The touch of a single wisp
of wool, less that you can detect on your skin, is enough to start
some tendrils responding. The organs of touch in humans can detect
a fine hair weighing 0.002 mg drawn across the skin. The sensitive
hairs of Drosera, the sundew, can detect a stimulus of
0.0008 mg, while Sicyos tendrils respond to 0.00025 mg,
which is eight times lighter than humans can detect. Not only
have plants the ability to sense what's going on, bus some do
it far better than we can.
The electrical nature of the stimulus has been demonstrated in
several ways. There are action potentials which can be measured
in a stimulated tendril, for one thing; and if an electrical signal
is actually fed to a tendril, it can itself induce coiling. The
pioneering experiments by the Italian physiologist Luigi Galvani
(1737 - 98) at the University of Bologna in the late eighteenth
century showed that electricity could stimulate frog muscle and
make it twitch. Now we have a made similar observations in the
tendrils of flowering plants.
There is a further comparison between plant and animal movement,
namely that plants can be anaesthetised much like humans. It has
been known for many decades that a dose of ether, chloroform,
or morphine can render a plant senseless.
From page: 209
. . . the most dramatic is the voodoo lily, Sauromatum guttatum.
. . it emits the odour of rotting meat, the perfect lure for flies,
and even heats up so that it perfectly imitates decomposing flesh.
When it is approaching maturity, the flower becomes warm to the
touch (some 15 degrees Celsius, 27 degrees Fahrenheit hotter than
the rest of the plant) and burns energy at an astonishing rate.
It is certainly comparable to the metabolic rates of a fast-moving
mammal.
From page: 212
In sunlight the leaves of the telegraph plant [Desmodium gyrans]
stick out at right angles to the stem, but in the hours of darkness
they hang straight down. . .
From page: 213 - 215
Simple folding movements in response to a general stimulus (like
light levels) rather than a specific stimulus (like touch) are
known as nastic movements, or nasties. . . It's an odd term .
. . and means 'folding'.
Thermonasties are the movements in plants caused by a change in
temperature. The familiar rhododendron evolved in high mountains
where low temperatures are common, and evolved a nasty to deal
with the cold. Normally the plant has outstretched leaves like
those of a laurel, but then the temperature falls below freezing
they curl inwards and roll up; each leaf also droops towards the
ground. In this way they can reduce the chance of frost damage.
Not only do Oxalis leaves close when rain falls on them,
but they respond to nightfall and remain closed during the hours
of darkness. In direct sunlight the plants respond by closing
their leaves, just as though it were raining. In this way they
can see when it's too bright, and can take action to prevent damage
to the leaves through overheating.
Although plants like light, and green plants need it to grow,
too much light is often as great a hazard. One of the simplest
but most dramatic examples is the compass plant Silphium
which grows on the US prairies. As flat new leaves grow, they
detect the direction of sunlight and develop in a north-south
alignment. All the leaves of every plant are parallel to each
other. This means that as the sun rises it shines directly onto
one side of a leaf, giving it the full benefit of the solar energy.
As the sun rises towards noon, it moves round so that it is shining
edge-on to the leaves which are thus protected from the full-
heat of the midday sun. During the afternoon, the sun moves round
so that it starts to shine on the opposite side of the leaves,
and they receive a second intake of solar energy. The leaves do
not move, but their careful orientation means that they can extract
the sun's energy throughout the morning and afternoon while avoiding
the risk of over heating during the heat of noon. The compass
plant manages to have a siesta through its curious design.
A sense of electricity
Plants have another sense of which we have little knowledge -
the ability to detect an electrical field. There has long been
a belief that a lawn become's greener before the torrential downpour
of a thunderstorm. This belief is not new. Early in the twentieth
century, experiments were carried out where high-tension cables
were stretched across a field of growing crops. The results showed
that the plant did indeed 'green up' in response to an electrical
field. During the 1990s, research at Imperial College, London,
located sensory cells within the plant which have the ability
to sense electricity. It is true - plants really do "green
up" in thundery weather before the rain starts.
The physiological purpose of this is complex. Before a dried-up
plant can fully benefit from rainfall, a great cascade of enzymes
leads to be set in train. The dried leaves need to return to an
active state of metabolism, ready to receive the water when it
comes. This takes time, and when rainfall is due, the sooner the
process starts the better. Plants have adapted to this fact. When
an electrical storm is approaching cells within the grass leaves
begin to mobilise their metabolic processes, ready for the rainfall.
The lawn really does turn green and, through the grass's extraordinary
senses, it does so before the first drops of rain begin to fall.
From page: 215
Solar input
Avoiding the sun may be important for some desert species, but
plants in cooler latitudes need to collect sunlight whenever they
can. In these species we find a regular daily movement as the
plants track the sun across the sky, turning to keep facing it
from dawn to dusk. Some plants are named for this legendary ability,
like the sunflower and the heliotrope (helios is the Greek
for 'sun'). The leaves: of cotton plants follow the sun, and Malvastrum,
a plant of the deserts of California, also has leaves which it
keeps facing the sun from dawn to dusk. These are tough species
well used to the rigours of a hot climate. Lupins like Lupinus
arizonicus follow the sun too, though they have more delicate
leaves and manage to avoid over exposure by turning their leaves
aside during the hours around noon.
Northern plants need sunlight more than any others, for it is
in short supply, and the flowers of the Arctic tundra need all
the light they can get. One garden plants which is found growing
wild in northern and Western Europe, Dyras octopetala,
shows how important it is to follow the sun. An experiment showed
what happened if you fix the flowers to stop them turning through
the day. In the plants whose flowers were free to follow the sun,
the internal temperature rose 1 degree Centigrade (1.8 degrees
Fahrenheit) higher than in those whose flowers were prevented
from moving. This is an important difference, for it turns out
that the size (and hence the viability) of the seeds is a function
of the internal temperature of the flower. Thus, the ability to
turn and face the sun can affect the chances of survival for plants.
In a few species, the behaviour depends on the conditions under
which the plant lives. An Australian plant grown as, forage crop
called siratro, Macroptilium atropurpureum, will orient
its leaves so that they face the sun when the ground is rich in
water. During conditions of drought, however, it changes its behaviour
and holds the leaves edge-on to the light (like the compass plant).
Clearly, this is meant to reduce the amount of evaporation from
the leaves when ground-water is in relatively short supply. These
mechanisms are present in a large number of plants. Even those
which do not actually turn to follow the sun are able to sense
the direction and strength of light, so that new leaves do not
overshadow older ones more than necessary. The mosaic of leaves
in trees, or on plants like ivy or Virginia creeper, is carefully
contrived to give each leaf a fair share of the light. In that
sense, all plants can see and can grow to optimise their benefits
from sunlight. Before concluding that this is a simple physical
mechanism, like a magnet attracting a pin, we need to understand
that this turning mechanism sometimes suggests that plants have
some form of active memory.
Plants become even more interesting when we consider not what
they do during sunlight, but how they behave in the dark. Many
of the species that follow the sun prepare themselves each night
for the dawn. They turn to face the direction where the sun is
due to appear. Malvastrum provides an intriguing example.
The leaves are kept facing the heat of the sun all day long, finishing
up at sunset facing west. After the sun goes down they resume;
a normal posture with the leaves spread out conventionally and
facing upwards. As dawn approaches the leaves turn again to face
the east, ready for the time when the sun will appear. It is not
unusual for sun-following plants to turn towards the direction
of dawn during the hours of darkness. An interesting experiment
would be to grow some plants in pots, and turn them through 180
degrees. That would show how quickly they could learn where the
sun was due to rise, and it would demonstrate whether they were
truly remembering the direction, or simply sensing a change in
the light.
From pages: 217 - 218
How plants see
The sense of light detection exists even in seeds. Many plant
seeds require sunlight to germinate. This explains the poppy fields
of the battlegrounds of the First World War. Within weeks of terrifying
ground attacks, the churned fields were cloaked in blood-red poppies.
. . The disturbance of the soil brought to the surface many seeds
which were stimulated to germination.
It was the sunlight which made the poppies grow. An old farming
tale suggests that fields ploughed at night grow fewer weeds.
Some recent research suggests that levels of weed growth can be
cut by 80 per cent by ploughing at night. This does not make immediate
sense to me - even if the seeds were exposed at night, they would
surely be stimulated by sunlight the next day. However, the facts
stand and more research will doubtless unearth the real answer
to plants and their reactions to light.
Light is the green plant's primary source of energy, so it is
natural that a plant should be able to detect light well enough
to ensure that it derives maximum benefit from the sun. Some of
the mechanisms of sense are simple. Plant growth hormones in stems
are more concentrated in dark tissues than in those exposed to
light. The effect of light on growth hormone seems to 'drive it
away'. The plant-growth hormones, auxins, stimulate cells to grow.
Imagine an upward-growing shoot, a simple rod of plant tissue.
If light shines onto the left of the shoot, then this lighter
side will continue to grow rather slower than before. The increase
in size of the tissues on the darker (right) side will therefore
tend to bend the shoot toward the light. Once illumination is
evenly detected on all sides, the growth will continue in a straight
line. In this way, a shoot will always grow towards the light.
From pages: 222 - 223
Plants and vision
There are light-sensitive chemicals in plants which do not take
part in photosynthesis, but give the plant its sense of vision.
Phytochrome, for instance, can sense the relationship between
red and far-red light, enabling plants to sense the presence of
their green neighbours. Another is riboflavin, which was proposed
as a light receptor as a result of some simple experiments. If
plant tissues are treated with potassium iodide, riboflavin (but
not carotene) is inactivated. In that state, the plant fails to
respond to light, which strongly supports the idea that riboflavin
acts as a sensory compound. Another example is phytochrome. Although
there has been much research into this potentially revealing area,
it is hard to tell the relationship between a light sensitive
pigment and the way a plant responds. This is where molecular
biology could hold the key to unravelling the mechanisms hidden
in the green plant's sense of sight. Riboflavin may have connections
with human life. It was originally known as vitamin B2 and is
essential for health. A lack of riboflavin in the human diet leads
to problems with the eye. Cataracts can develop, the eye can become
reddened, and a sense of itching or soreness develops. People
deficient in riboflavin find bright light unbearable, and this
photophobia can be so severe that they cannot endure even normal
daylight. Such interesting coincidences between human sight and
the photoreceptors of the plant kingdom remind us of the universality
of the senses. The notion that perception is confined to humans,
or even to the animal kingdom, can no longer be sustained.
From pages: 239 - 241 Languages of plants
Plants have a vast vocabulary of signals and responses. They can
detect the signs of a change in their environment and adjust their
metabolism to anticipate its effects. They have finely tuned senses
for signs of environmental stress which can compensate for its
effects. Plants are able to detect the signals that herald a change
in the leafy canopy above them, and alter their architecture in
order to retain their fair share of light. Plants can detect changes
in the total amount of light reaching them, and can also sense
changes in the spectrum of the light. The most obvious method
is for the plant to respond to the amount of light energy, sometimes
even by twisting the leaf to avoid too much light, and the extent
of photosynthesis within the leaf. However, we also believe that
plants have specific light sensors which act as "yes'. They
detect the nature and extent of the light, but consume very little
of its energy.
This may be regarded as a sense of 'sight', quite distinct from
a mere response to the effects of light energy on the rate of
metabolism of the plant cell. Plants detect light in colour. They
have sensors specifically for ultraviolet, blue, red and far-red
radiation. These sensors provide them with a rich and detailed
impression of their situation. Plants use the ratio of red to
far-red radiation to control their rate of growth. By changing
the speed at which stems elongate, a plant seems to be able to
avoid future deleterious effects on its development. In particular,
changes in light quality can forewarn the plant about future changes
before any shading by neighbours has occurred, and we have seen
that plants detect wind, too. Changes in calcium ions, Ca+2, trigger
alterations in gene expression which modify the way the plant
grows. The metabolism, the expression of genes and the rate of
growth of the plant are an integrated response to all this sensory
input.
Meanwhile, plant roots adjust to the availability of nutriment,
and favour areas where nourishment is most abundant. Their sensing
of the availability of food and water allows the plant to adjust
its rate of nutriment uptake. Not only can they sense gradients
of moisture in the soil, but they change their rate of growth
in the presence of nearby roots. It seems that they are able to
control their growth in order to avoid too much competition for
scarce supplies of raw materials. There are mycorrhizal associations
between plants and fungi, in which a fungus colonises the roots
of a host plant (sometimes even penetrating inside the host plant
cells, but causing no disease). These relationships bring a number
of benefits. Not only do the fungi process wastes in the soil
and recycle them as foodstuffs for the growing plant, but the
interchange of compounds between fungus and plant provides many
opportunities for communication and the transmission of warning
signals. Plants have many of the senses possessed by humans. They
have sight, as far as they need it; they have a sense of touch
(sometimes to an extraordinary degree); can sense temperature,
and - through gravity - they can tell 'up' from 'down', or 'left'
from 'right'. Twiners can (usually) tell clockwise from anti clockwise.
Plants can remember stimuli and tell one form of stimulus from
another. They can communicate, and they cooperate to survive.
If plants required more intelligence, they would have developed
it. As it is, their senses and the limits of their sentience are
exactly what they require. Some of the senses in the plant world
are already more highly developed than ours (the sense of touch,
for example). No longer should science regard a green plant as
a simple organism which endures what it must, and adjusts like
a chemical system. We owe plants respect, for on green plants
we all rely for survival. They are not our subjects; plants are
our cousins."
To read about the book "The Secret Language of Life"
by Brian J. Ford, and to possibly purchase it at Amazon Books
click here.
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booklight reviews if you do. Reviewed by David Baillie, Harmony
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